Accurately produced non-explosive gas mixtures are required in applications as diverse as welding, lasers, commercial and industrial processing, and medical surgery, to name just a few examples. These applications require that gases drawn from high-pressure sources be combined in specified proportions before being discharged at lower pressures for immediate use. It is necessary that the specified proportions of the gases in the mixture be constantly maintained. In addition, the typical use requires intermittent flow, that is, the gas flow is often stopped and started as required by the process.
Presently, when the flow of a mixed gas is interrupted, there is an increased probability that the mixture ratio of the gases is measurably altered for several reasons. For example, intermittent flow adversely affects mixture ratio, the result of incremental errors caused by pressure changes in volumes of gas between upstream control valves and their respective flow control orifices, or differences in individual valve response times, when the flow is started or stopped. In addition, separate gauges are typically used to monitor each inlet port. With multiple gauges, mis-calibration, or the differences between working tolerances of the gauges, often accounts for mixing errors. More exacting and delicate applications have a low tolerance for mixture errors associated with intermittent flow or multiple gauges.
Attempts have been made to overcome the problem of maintaining a mixture ratio under interrupted flow and changes in operating pressures. For example, it is known to construct a mixing device which closes independent of the pressure in the feeding lines, as the pressure differential between the mixing chamber and gas discharge chamber drops below a set value. Also, it is known to absorb pressure fluctuations within a gas-mixing device by providing surge dampers and back pressure valves. In addition, it is known to provide a bi-stable diaphragm which allows the gas from a high pressure source to replenish the pressure within a receiver, by opening and closing in response to the receiver's lower pressure level. However, the existing devices do not maintain constant gas ratios under both sonic and subsonic flow; do not perform all functions within a self-contained single mechanical device; do not maintain constant gas ratios when a part of the gas-mixer is depressurized; and generally employ separate pressure gauges for each supply line.
U.S. Pat. No. 4,699,173 (Rohling) describes a mixing device which seeks to maintain a constant ratio between gases during interrupted flow. Rohling includes a gas discharge chamber separated from a mixing chamber by a pressure-sensitive, spring-mounted assembly that includes a connecting channel. There, the connecting channel is held in the open position by the assembly until the pressure in the discharge chamber rises. The assembly then closes the connecting channel to stop the flow of mixed gases. After the connecting channel is closed, increased pressure in the upstream supply lines causes upstream pressure reducing valves to close. The assembly opens the connecting channel when the pressure in the discharge chamber drops below a predetermined level.
The mixing device disclosed in U.S. Pat. No. 4,576,159 (Hahn) describes a mixing chamber which operates together with pulsed control valves to produce a regulated flow of mixed gases. In addressing the problem of mixture ratio errors, Hahn teaches that the outlet orifices of the pulsed valves should be mounted so as to be in good thermal communication with one another. As explained, this is important to minimize the effect of relative thermal expansion or contraction of the orifices which can render the relative flow rates of two gases inaccurate.
U.S. Pat. No. 5,411,051 (Olney et al.) is directed to a means for automatically replenishing a receiver, such as a deflated tire, from a high pressure reservoir. To that end, Olney teaches a bi-stable diaphragm which moves between two stable positions, respectively opening and closing the air flow passage from the high pressure reservoir. When the tire pressure falls below a threshold level, the increased pressure differential across the diaphragm causes it to flex and lift, thereby opening an air passage between the tire valve stem and high pressure reservoir. By way of alternative embodiments, the bi-stable diaphragm may contain a magnetic element, be constructed of pre-stressed metal, or be configured to include bellows.
The primary thrust of the prior art devices is to supply air or combine gases drawn from elevated pressure sources, and discharge the mixture at a lower pressure. Because conventional mixers usually start and stop flow with a control valve upstream or downstream from the control orifices, intermittent flow adversely affects the mixture ratio as a result of the pressure changes between the control valves and their respective flow control orifices. Likewise, because conventional mixers use separate gauges to monitor each inlet port, mis-calibration or the differences between working tolerances of the gauges may create mixing errors. Finally, the configuration of the prior art devices do not lend themselves to a single, self-contained and compact device.
Thus, there is a need in the art for a gas mixing device which maintains mixture ratios under both steady state and intermittent flow conditions; eliminates mixture errors under both sonic and subsonic flow conditions; uses only a single pressure gauge to determine all inlet port pressures; is easy to dismantle and repair; lends itself to a compact and unobtrusive configuration; is simple in design; and is rugged enough to be compatible with a variety of environments.